It relates more precisely to event detection applications for a M2M communication (for “Machine to Machine”) in a M2M network comprising sensors and at least one collector resulting from the occurrence of an event detected by a sensor in the network. In a M2M network, an event is associated with the observation of data of one or more types. When a condition relating to at least one item of such observed data is satisfied, an event is generated and signalling of this event is transmitted to the collector. The detection of the event can be related to an alert within a security application. The event can occur consecutive to the passing of a sensor measurement below or above a threshold value, for example. The moment when the signalling of the event is sent can be defined by means of a timer and a predefined value. Thus, when the timer reaches the predefined value, sending the signalling is triggered.
The signalling of an event can therefore be periodic, as in the case of a periodicity for taking measurements from a sensor, or asynchronous when it is transmitted consecutive to the occurrence of a detected event, like the passage of a sensor measurement below or above a threshold value, as mentioned above.
Such communications between sensors and a collector generally have constraints with regard to security of operation and security of information carried. In fact, because of the nature of the communication, in the form of radiofrequencies, any message transmitted by one of the entities, sensor or collector, can be intercepted by a malicious third party.
At present, cryptographic methods make it possible to make information contained in messages exchanged secure. They propose for example the use of encryption/decryption devices based on at least one secret key.
However, in the case of presence detection systems inside premises for example, the simple fact of sending a message consecutive to the detection of an event, already constitutes information.
For example, a presence detection system installed in the home of a user can comprise a set of sensors distributed through a set of rooms in the home, the sensors being suitable for communicating with a collector. Thus, when one of the sensors detects a presence in a room, it can transmit an encrypted message to the collector. A malicious third party located outside the home can, by means of a radio listening device, detect the transmission of the message and deduce the occurrence of a detection by a sensor inside the house. This knowledge can then be used by the third party to commit malicious acts. In fact, upon the detection of a presence and interception of the signalling, the malicious third party can decide to commit a malicious act or not. Consequently, the security and confidentiality of the system risks being compromised.
The detection system therefore does not make the communications between these elements transparent to a malicious third party.
A solution for making the detection of an event by a network of sensors transparent would consist of making the communications strictly periodic and, in each period, sending an encrypted message containing either information relating to the detection of an event or information relating to no detection.
However, in order to obtain a level of responsiveness close to that of asynchronous detection systems at a pace set by the occurrence of events, systems detecting events by sending periodic messages must have short periods, which generates a high energy consumption. Also, these systems are easily intercepted and jammed. Finally, the management of the periodic sending of messages in parallel to the individual detection of an event is complex.
Another solution consists of using a transmission of ultra wide band messages (UWB), which consists of a low-energy emission in a wide spectrum of frequencies. Thus, the message sent is concealed beneath a noise threshold but can be picked up by a receiver when the latter is synchronized with a transmitter of such messages. A UWB transmission mode, called TH-UWB (for “Time-Hopping Ultra Wide Band”) consists of transmitting ultra wide band signals, of GigaHertz (GHz) order, of very short duration, of the order of a nanosecond, in an irregular manner, the moments of transmission being determined by a spreading code, such as a time skip code for example. The transmissions of pulses are therefore spaced apart with respect to one another, the spacings being provided by the time skip code. Since this skip code is shared by the transmitter and the receiver, the receiver can open demodulation windows at the moments of arrival of the pulses. In a variant, it is possible to detect the pulses, measure the time differences between the pulses received and compare these differences with the skip code to find the sequence transmitted.
However, techniques such as that known as a “dirty template” make it possible for a third party to find a recurrent time signature in a signal transmitted and thus to find a form of synchronization, without having any knowledge of the skip code used. The security inherent in detection systems using the transmission of radio frequencies therefore does not make it possible to prevent the interception of information by a third party.